Twin walled polycarbonate sheeting which has a plurality of spacing webs between the walls is relatively new in the moulding industry. Due to its lightweight, and excellent light transmission and insulating properties it makes it very useful in the roofing industry, as well as other industries. Until recently the moulding of such materials has not been possible with conventional moulding apparatus due to the temperature differential resulting on the walls and the cracks and bubbles appearing on the surfaces of the moulded sheet. Unfortunately, whereas simply moulding of flat sheets has now been achieved, the moulding of an arcuate configuration, such as a barrel vault or bull noze as commonly known, in the industry has not yet been possible.
It is therefore an object of the present invention to overcome or substantially ameliorate this disadvantage.